Mutations in the KRAS oncogene occur in 40% of colorectal cancers (CRC), and despite extensive investigation, KRAS mutated CRCs remain resistant to available targeted therapy strategies. Mutant KRAS has been linked to activation of multiple signaling pathways that promote cancer growth and survival including the MEK-ERK, PI3K and NF-?B pathways. Suppressing critical downstream signaling pathways, either alone or in combination, have evolved as promising treatment strategies. This proposal consists of innovative translational laboratory and clinical studies that focus on developing novel strategies to treat KRAS mutant CRCs. These investigations have already begun to yield strategies that will be assessed in clinical trials in this proposal. Our overarching aim is to substantively advance the treatment of KRAS mutant CRCs in this funding period. We have conducted comprehensive signaling studies, genetic screens, and drug screens to identify and validate genes downstream from KRAS whose expression is essential to the growth and survival of KRAS mutant cancers. Our analyses of 1000 cell lines treated with >200 drugs revealed that MEK inhibitors are the most effective class of agents against KRAS mutant CRC cell lines. However, single agent MEK inhibition appears minimally effective in clinical trial; as such we will develop novel combination strategies that utilize MEK inhibitors as a backbone for KRAS mutant CRC. We recently discovered that combining an IGF-IR inhibitor, which has minimal activity as a single-agent, with a MEK inhibitor is highly effective in KRAS mutant CRC cell lines in vitro and in vivo, leading to the development of a soon to open phase l/ll clinical trial of this combination. To discover additional combinatins, we developed an innovative pooled shRNA screen to identify MEK inhibitor-based combinations for KRAS mutant CRCs. One gene identified in the screen was BCL-XL, and initial studies demonstrate that inactivation of BCL-XL potently synergizes with MEK inhibitors both in vitro and in vivo, which we will further explore in the laboratory and in planned clinical trials. Finally, we will build on our preliminary data demonstrating that Tank Binding Kinase (TBK1) activity is required for KRAS mutant cell survival. We will examine combined TBK1 inhibitors alone and combined with MEK inhibitors. The studies in this proposal will span cell lines, genetically engineered mouse models and clinical trials to identify novel therapeutic strategies for the trea treament of KRS mutant CRCs.